Q-switching is a method for obtaining single laser pulses of very high power by protracting the period of population inversion of electrons in excited states just prior to emission. Extending the period of population inversions typically can be achieved acousto- or electro-optically by use of shutters, mechanically (with an orthogonal mirror or rotating mirror), or by use of saturable absorbers (in the form of dyes or doped crystals).
The term “Q-switching” is a reference to the fact that a “Q-factor” or “Quality factor,” which can be defined as ν/Δνc, where ν is cavity resonance frequency, and Δνc is cavity linewidth, shifts from a very low value to a very high value during laser pulse emission. More specifically, population inversion of electrons is extended by blocking emission from the laser cavity. At the time a laser pulse is to be emitted, the blockage is removed, thereby causing the threshold gain of electrons to be deliberately and suddenly reduced. Population inversion is much higher than the threshold gain value, and actual gain greatly exceeds cavity losses. As a result, the excited states are quickly depopulated, causing energy to be discharged in a single laser pulse. The sudden discharge causes actual gain to be reduced to a point below the threshold value, thereby terminating the pulse.
Saturable absorber Q-switches operate passively, whereby absorptivity of the laser wavelength decreases with increasing irradiance until “bleaching” occurs. Population inversion increases until the Q-switch is bleached, at which time the threshold value is reduced, resulting in a laser pulse. Passive Q-switches typically are easy to implement relative to other mechanisms. Historically, examples of saturable absorber Q-switches are dyes, such as bis 4-dimethyl aminodithiobenzyl-nickel (BDN) dissolved in 1,2 dichloroethane for Nd:YAG lasers, and gases, such as SF6 for CO2 lasers.
More recently, solid state Q-switches have been employed that include crystals doped with tetrahedrally coordinated Co2+ ions as a tunable laser source in wavelengths that range from about 1.5 to about 2.3 μm. Among the crystals that have been doped with Co2+ ions for 1.34 μm Nd3+:YAlO3 and 1.54 μm-Er3+: glass lasers are Y3Al5O12, Y3Sc2Ga3O12, LaMgAl11O19, MgAl2O4 (MALO) and ZnSe. MgAl2O4 crystals, otherwise known as spinel, include tetrahedral and octahedral positions. Co2+ dopant ions displace Mg2+ ions from tetrahedral positions of the crystal. The amount of Co2+ ion dopant in MgAl2O4 crystals typically ranges from about 0.0003 atomic weight percent to about 0.05 atomic weight percent. However, the frequency of the peak emission of doped solid state passive Q-switches typically is not affected by the amount of dopant. Further, the efficiency of a Q-switch (and, thus, the power of the laser pulse) is significantly diminished if it does not have an absorption band that matches the lasing transition. For example, spinel having the empirical formula of MgAl2O4 and doped with Co2+ typically has an absorption band (4T1 spectrum) of about 1536 nanometers (nm), whereas the lasing transition of Er:Yb:glass lasers is about 1540 nm. Generally, the efficiency of cobalt-doped spinel Q-switches in Er:Yb:glass and other lasers is limited by the difference in specific absorption bands from the lasing transition wavelengths of such lasers.
Therefore, a need exists to significantly diminish or eliminate the above-mentioned problems of cobalt-doped saturable absorber Q-switches.